1-14 CAUTION NEVER connect a 400-Hz synchro to 60-Hz voltage. The reduced impedanceresults in excessive current flow and the windings quickly burn out. Q-16. What type of equipment normally uses 26-volt 400-hertz synchros? OPERATING TEMPERATURES AND SPEEDS Standard synchros are designed to withstand surrounding temperatures ranging from -67º F to +257º F (-55º C to +125º C) at the terminal board. Prestandard synchros operate in a range of -13º F to +185º F (-25º C to +85º C). When a synchro is energized and not loaded, its temperature should stay within prescribed limits. Loading an energized synchro causes it to generate more heat. Similarly, overloading causes a synchro to generate much more heat than it would under normal loading conditions and could possibly result in permanent synchro damage. To meet military specifications, all standard synchros must be capable of continuous operation for 1,000 hours at 1,200 revolutions per minute (rpm) without a load. A prestandard synchro has one of two specifications, depending upon its use in a data transmission system. Low-speed prestandard synchros must be capable of continuous operation for 500 hours at 300 rpm without a load. Low-speed prestandard synchros must be capable of continuous operation for 1,500 hours at 1200 rpm without a load. Q-17. When will a synchro generate more heat than it is designed to handle? THEORY OF OPERATION Synchros, as stated earlier, are simply variable transformers. They differ from conventional transformers by having one primary winding (the rotor), which may be rotated through 360º and three stationary secondary windings (the stator) spaced 120º apart. It follows that the magneticfield within the synchro may also be rotated through 360º. If an iron bar or an electromagnet were placed in this field and allowed to turn freely, it would always tend to line up in the direction of the magnetic field. This is the basic principle underlying all synchro operations. We will begin the discussion of synchro operation with a few basic points on electromagnets. Look at figure 1-8. In this figure, a simple electromagnet is shown with a bar magnet pivoted in the electromagnet's field. In view A, the bar is forced to assume the position shown, since the basic law of magnetism states that like poles of magnets repel and unlike poles attract. Also notice that when the bar is aligned with the field, the magnetic lines of force are shortest. If the bar magnet is turned from this position and held as shown in view B, the flux is distorted and the magnetic lines of force are lengthened. In this condition, a force (torque) is exerted on the bar magnet. When the bar magnet is released, it snaps back to its original position. When the polarity of the electromagnet is reversed, as shown in view C, the field reverses and the bar magnet is rotated 180º from its original position.